The field of the invention is that of wideband antennas with hemispherical or near-hemispherical radiation patterns. More specifically, the invention relates to helical antennas of this type.
The antenna of the invention is found especially in applications of satellite mobile communications between fixed and/or mobile users of all types, for example aeronautical, maritime or terrestrial communications. In this field, several satellite communications systems are implemented or are now being developed (these include the INMARSAT, INMARSAT-M, GLOBALSTAR, and other systems). These antennas are also valuable in the deployment of personal communications systems (PCS) using geostationary satellites.
The systems are designed to provide terrestrial users with new communications services (multimedia, telephony and other services) through satellites. By means of geostationary or orbiting satellites, they provide global terrestrial coverage. They have to be similar to terrestrial cellular systems in terms of cost, performance a size. Thus, the antenna located in the user""s terminal is a key factor in size reduction.
Systems of this kind are described especially in Howard Feldman, D. V. Ramana:  less than  less than An introduction to Inmarsat""s new mobile multimedia service greater than  greater than , Sixth International Mobile Satellite Conference, Ottawa, June 1999, and J. V. Evans:  less than  less than Satellite systems for personal communications greater than  greater than , IEEE A-P Magazine, Vol. 39, No. 3, June 1997.
For all these systems, which provide links with geostationary satellites, the very different values of incidence of the signals received or sent require that the antennas should possess a radiation pattern with hemispherical or near-hemispherical coverage. Furthermore, the polarization must be circular (left-hand or right-hand) with a ratio below 5 dB in the useful band.
More generally, the invention can be applied in all systems requiring a small-sized antenna, the use of a very wide band and circular polarization.
In these different fields of application, the antennas must often have the above characteristics either in a very large bandwidth of about 10% or in two neighboring sub-bands corresponding respectively to reception and to transmission. It is also essential that the size and weight should be reduced to the greatest possible extent.
The invention can be applied especially to quadrifilar antennas.
A quadrifilar antenna is formed by four radiating strands. An exemplary quadrifilar antenna is described in detail in A. Sharaiha and C. Terret, xe2x80x9cAnalysis of quadrifilar resonant helical antenna for mobile communicationsxe2x80x9d (IEE Proceedings H, vol. 140, no 4, August 1993).
According to this embodiment, the radiating strands are printed on a thin dielectric substrate and then wound about an RF-transparent cylindrical support. The four strands of the helix are open or short-circuited at one end and electrically connected at the other end.
This antenna requires a power circuit that excites the different antenna strands by signals having the same amplitude in phase quadrature. This function may be performed by means of structures comprising 3 dBxe2x88x9290xc2x0 couplers and a hybrid ring. This assembly can be made in printed circuit form and placed at the base of the antennas. Thus, a simple but bulky power supply system is obtained.
As mentioned further above, it is desirable for the antenna (including its supply) to be as small-sized and lightweight as possible.
Several solutions have been proposed to this end.
For the power supply system, a solution has been proposed based on the making of three hybrid couplers designed as semi-localized elements and printed in the prolongation of the antenna. This technique is described especially in the patent FR-96 03698, filed on behalf of the present applicant.
The antenna itself has three known improvements in particular.
A first approach is described by B. Desplanches, A. Sharaiha, C. Terret in  less than  less than Parametrical study of printed quadrifilar helical antennas with central dielectric rods greater than  greater than  (Microwave and Opt. Technol. Letters, Vol. 20, No 4, Feb. 20, 1999). This solution of miniaturization augments the permittivity of the cylindrical support around which the substrate is wound.
This technique reduces the height by about 30 percent. It is also very simple to make. However, it has the drawback of reducing the bandwidth. Furthermore, it is costly.
According to a second solution, the height of the antenna may be reduced by cutting each strand into two distinct parts having a length of about xcex/4 with a symmetry with respect to the middle of each strand. This technique is described especially in the article by D. F. Filipovic, M. Ali Tassoudji, E. Ozaki:  less than  less than A coupled-segment quadrifilar helical antenna greater than  greater than  (MTT-S Symposium on technologies for wireless applications, Vancouver, Canada, 1997).
Again, this gives a satisfactory reduction in height (by 28.4% in the example given), without any modification in the radiation pattern and the ratio of ellipticity. Furthermore, the structure proves to be simple.
By contrast, the bandwidth is reduced to 3% for a SWR value less than 2. Furthermore, an antenna of this kind requires difficult adjustments of the coupling between the active strands and the passive strands.
A third proposal for reducing the height of the printed quadrifilar helix (PQH) antenna is to wind each strand of the helix according to a non-linear equation as described in M. E. Ermutlu:  less than  less than Modified quadrifilar helix antennas for mobile satellite communication greater than  greater than  (IEEE APS Conference on antennas and propagation for wireless communications, Piscataway, N.J., 1998). This approach can give a size reduction of 14%.
However, this technique introduces a deterioration of the ratio of ellipticity throughout the coverage.
In other words, the known techniques used to reduce the height of the antenna show major defects in terms of characteristics. The operation of reduction leads to the deterioration of the bandwidth and/or of the ratio of ellipticity.
Furthermore, as mentioned further above, it is often desirable to have a large bandwidth and/or bandwidths corresponding to transmission and reception respectively.
The patent FR-89 14952 filed on behalf of the present applicant describes a type of antenna particularly suited to such applications.
This antenna, known as a printed quadrifilar helix (PQH) antenna, possesses characteristics similar to those laid down by the criteria set forth, in a frequency band generally limited to 6% or 8% for an SWR of less than two. A wider band operation can be obtained by using two-layer PQH antennas. These antennas are formed by the concentric xe2x80x9cnestingxe2x80x9d of two electromagnetically coupled coaxial, resonant quadrifilar helixes. The assembly works like two coupled resonant circuits whose coupling separates the resonant frequencies. Thus, a two-layer, resonant, quadrifilar helix antenna, according to the technique described in FR-89 14952, is obtained.
This technique has the advantage of requiring only one power supply system and of enabling dual-band and wideband operation.
However, it has the drawback of requiring the manufacture of two printed and nested circuits and of offering only a small bandwidth in each sub-band.
A quadrifilar antenna is formed by four radiating strands. An exemplary embodiment is described in detail in A. Sharaiha and C. Terret, xe2x80x9cAnalysis of quadrifilar resonant helical antenna for mobile communications,xe2x80x9d (IEExe2x80x94Proceedings H, vol. 140, No. 4, August 1993).
According to this embodiment, the radiating strands are printed on a thin dielectric substrate and then wound about an RF-transparent cylindrical support. The four strands of the helix are open or short-circuited at one end and electrically connected at the other end.
This antenna requires a power circuit that excites the different antenna strands by means of signals having the same amplitude in phase quadrature. This function may be performed by means of structures comprising 3 dBxe2x88x9290xc2x0 couplers and a hybrid ring. This assembly can be made in printed circuit form and placed at the base of the antennas. Thus, a simple but bulky power supply system is obtained.
As mentioned further above, it is desirable that the antenna (including its supply) should be as small-sized and lightweight as possible, and that it should cost as little as possible.
Several solutions have been proposed in order to reduce the dimensions of the antenna and of its power supply system. Among other examples, we may cite for example the solutions presented in the FR-96 03698, filed on behalf of the present applicant and in B. Desplanches, A. Sharaiha, C. Terret,  less than  less than Parametrical study of printed quadrifilar helical antennas with central dielectric rods greater than  greater than  (Microwave and Opt. Technol. Letters, Vol. 20, No 4, Feb. 20, 1999).
The invention is aimed especially at overcoming the different drawbacks of the prior art.
More specifically, it is a goal of the invention to provide a small-sized resonant helix antenna having a very large bandwidth and/or two bandwidths covering the transmission band and the reception band of a communications system.
In particular, it is a goal of the invention to provide a helix antenna of this kind whose size, performance and cost price are adapted (and hence at least similar) to the portable terminals of terrestrial cellular systems. In this approach, the size and the weight of the antenna are crucial aspects.
According to another aspect, it is a goal of the invention to provide a resonant helix antenna having a very large bandwidth and/or two bandwidths covering the transmission band and the reception band of a communications system.
In particular, it is a goal of the invention to provide a helix antenna of this kind having a major bandwidth (greater than the bandwidth obtained in the prior art) in each sub-band, when two sub-bands are planned.
It is another goal of the invention to provide an antenna of this kind whose size, performance and cost price are adapted (and hence at least similar) to the portable terminals of terrestrial cellular systems.
Another goal of the invention is to provide characteristics similar or superior to those of the double-helix antennas (which are more complicated to make) with a single helix.
These goals, as well as others that shall appear here below, are achieved according to the invention by means of a helix antenna comprising at least one helix formed by at least two radiating strands, at least one of said strands of which is formed by at least two segments, the pitch angles of at least two of said segments being different and determined randomly or pseudo-randomly by the global optimization means.
This novel and inventive approach provides for a satisfactory reduction in the size of the antenna (as compared with a classic antenna having strands with a constant pitch angle), the manufacture and cost price remaining identical.
Preferably, said strands are printed on a substrate. This mode of manufacture, which is known per se, is both simple and efficient.
According to an advantageous embodiment of the invention, at least one of said helixes is a quadrifilar helix, comprising four strands.
Preferably, the strands forming a helix all have the same geometrical characteristics. However, in certain particular embodiments, strands that are different from one another may be envisaged.
In general, the segments may have any lengths whatsoever, and these lengths may be identical or different. Similarly, there may be any number of segments per strand, and the pitch angle of each segment may be any angle (from 0xc2x0 to 90xc2x0).
The invention also relates to a method to determine the pitch angles of segments of strands of a helix antenna as described here above. A method of this kind advantageously implements a global optimization step in which pitch angle values are selected by:
(i) randomly or pseudo-randomly determining possible pitch angle values;
(ii) repeating the step (i) so long as said possible pitch angle values cannot be used to obtain a radiation pattern in terms of main and crossed polarization contained in a predetermined template.
This method can be used in particular to implement a global optimization program belonging, for example, to the group comprising simulated annealing and the genetic algorithm.
According to another aspect of the invention, it is advantageously planned that at least one of said segments of at least one of said strands will have a variable width.
The antenna thus obtained has a wider bandwidth (in one or two sub-bands) than the classic antenna with strands of constant width, hereinafter called a reference antenna, without increasing the complexity of manufacture or the cost price.
It must be noted that this aspect of the invention can also be applied to antennas whose strands more conventionally comprise a single segment.
According to an advantageous embodiment of the invention, the width of said segments, or segments of variable width, varies monotonically between a maximum width and a minimum width.
Advantageously, said segments of variable width are such that the width of said segments to which they belong varies monotonically between a maximum width (D1) and a minimum width (D2).
Preferably, the end having said maximum width is connected to a feeder line of a power supply circuit, the end having said minimum width being open.
According to a first embodiment of the invention, the width of said strand or strands of variable width varies regularly.
According to another embodiment, said width may follow a law belonging to the group comprising:
linear laws;
exponential laws;
double exponential laws;
stepped laws.
According to another approach, it can be planned that the width of said strands or said strands of variable width varies non-regularly.
Preferably, the dimensions of said strands are determined so as to provide a large bandwidth greater than 8% (and more generally greater than that of the reference antenna with constant-width strands) for an SWR of less than 2.
According to an advantageous embodiment of the invention, the dimensions of said strands are determined so as to give a double bandwidth.
As already mentioned, the bandwidths of each sub-band are greater than that of the reference antenna.